I. Field of the Invention
This invention relates to a heated fuser device as is commonly used in xerographic copying machines, and more particularly to a heated fuser roll whose heating elements are axially disposed along the roll and are formed of a material that enables the fuser to be temperature-self regulating.
II. Description of the Prior Art
In a typical xerographic process a photoconductor comprising a photoconductive composition coated on a rigid or flexible substrate is uniformly electrostatically charged in the dark, and then exposed by being illuminated in an image pattern in accordance with graphic material on an original document. The photoconductor becomes discharged in the areas exposed to the illumination, but retains its electrostatic charge in the dark areas, which areas correspond to the graphic material on the original document. The resulting electrostatic latent image is developed by depositing on the photoconductor a finely divided electrostatically attractable developing material (toner). The toner will normally be attracted to those areas on the photoconductor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This visible image of developing material is then transferred to a support surface, such as plain paper or any other suitable substrate, to become the ultimate copy. Any residual developing material remaining on the photoconductor is cleaned and the photoconductor is reused as described above for subsequent copies. The toner image that was transferred to the plain paper is then fixed thereto. Since the developing material is heat fusible, application of sufficient heat to the paper causes the developing material to melt and be fused into the paper so as to be permanently affixed thereto.
One very basic approach to fusing in a xerographic copying machine is the use of the so-called hot roll pressure fuser apparatus. Typically, in this apparatus, the paper with the toner image thereon is passed between a pair of opposed rollers, at least one of which is heated. Generally, the heated roll is formed of a hollow cylinder having a radiant heater, such as an infrared lamp or a halogen lamp, centrally located within the cylinder to heat the roll, in series with a bimetal thermostat. A typical example of this type of heated fuser roll is illustrated in U.S. Pat. No. 3,637,976. During operation of the fusing apparatus, the paper to which the toner images are electrostatically adhered is passed through the nip formed between the rolls with the toner image contacting the fuser roll to effect heating of the toner image within the nip. Fusing is enhanced by the second roll or pressure roll as it is commonly called as the result of a biasing force which forces the rolls into engagement. The thermostat intermittently interrupts the current flow as the roll temperature reaches a predetermined value. The roll then cools to some lower temperature whereupon the thermostat restores the current, and the roll heats up again.
Many of the problems that occur with the use of a hot roll-pressure fusing apparatus are in the heated fusing roll. In particular, these problems relate to the means employed for heating the fuser roll and its control. For example, in many of the known hot-roll fusers it is extremely difficult to maintain a constant temperature at the nip of the rollers where the actual fusing of the toner occurs, and where temperature control is critical. Temperature control is difficult because (1) it is difficult to sense the temperature in this region: (2) thermal lag, i.e., the responsiveness of roll temperature under varying demands of thermal output; and (3) there are both different machine modes, i.e., standby, off, continuous operation, and different size papers to contend with. The type of thermostat control as described above is conspicuously oscillatory in nature. The thermostat, by necessity being situated on the circumference of the roll in order to control the temperature of that surface, is relatively remote from the heater and, thus, the temperature fluctuations are usually significant. Reductions in this aforesaid differential temperature characteristic requires extensive and expensive proportional feedback control means. In addition to these problems, radiant-type heated fuser rolls generally require very high heating temperatures for the heating element to enable the roll temperature in the nip of the rollers to be high enough to melt the toner. The use of these high temperatures can result in deterioration of the fuser roll.
Examples of other miscellaneous type heated rolls exhibiting many of the problems as outlined above are illustrated in U.S. Pat. Nos. 3,471,683, 3,720,808 and 4,100,397.